When cutting a metallic work with a laser beam, the laser beam is condensed through a condenser lens into a spot of 100 μm (micrometers) to 500 μm (micrometers) to increase energy density and instantaneously heat the work to a metal melting point of 1500 degrees or over so that the work melts or sublimates. At the same time, an assist gas is fed to remove melted material and cut the work. When the work is a thick mild steel sheet (carbon steel sheet), an oxygen gas is used as the assist gas to generate oxidization reaction heat and utilize the heat as well for cutting the work.
A laser beam of a one-micrometer waveband from a solid-state laser or fiber laser realizes a very high optical energy absorptance on a metallic work compared with a laser beam of a ten-micrometer waveband from, for example, a CO2 laser. If the one-micrometer waveband laser beam is used with an oxygen assist gas to cut a mild steel sheet work, a melt width on a top face of the work widens more than necessity to impair kerf control. In addition, excessive burning (self-burning) may occur to deteriorate the quality of the laser cutting.
Tests were conducted to compare changes in kerf width on a top face of a work between the CO2 laser and the fiber laser by equalizing spot diameters of laser beams emitted from the lasers, by employing the same laser processing conditions (such as laser output and oxygen gas pressure), and by changing a focus position of each laser beam in a range of 0 mm to 6 mm from the top face of the work. As illustrated in FIG. 2(A), the CO2 laser demonstrated similar changes in both the kerf width and the focus position from the top face of the work. On the other hand, as illustrated in FIG. 2(B), the fiber laser demonstrated larger changes in the kerf width than in the focus position. Namely, the fiber laser provides greater thermal effect on the work than the CO2 laser.
This means that the laser beam of a one-micrometer waveband from the solid-state laser or fiber laser has a very high energy density in the vicinity of the center of the laser beam, and therefore, achieves a very high optical energy absorptance on a work. When cutting a work with a laser beam according to a required cut width, the laser beam is condensed to have a required spot diameter. At this time, if an oxygen gas is used as an assist gas for cutting the work, the work will easily cause self-burning to expand a melt width on a top face of the work (for example, a mild steel sheet) wider than the required cut width. Then, it is difficult to properly conduct kerf control or stabilize the quality of the cut work. It is required, therefore, for the laser cutting work using the fiber laser and oxygen assist gas to realize the same cutting quality as that realized by the CO2 laser.
For this, various tests were made and it was found that forming a laser beam of the fiber laser into a ring beam and cutting a work with the ring beam provide the same effect as that provided by the CO2 laser.
A related art that forms a laser beam of the fiber laser into a ring beam and cuts a work with the ring beam is disclosed in WO2010/095744A1 (Patent Literature 1).